The fact that the highly irradiated immunizing schistosome cercariae die in skin of immunized hosts and can induce a strong acquired immunity against schistosomiasis at the first line of defense has been well confirmed. Recently, however, conflicting results that both attrition of the highly X-irradiated cercariae and the challenge cercariae are not in the skin but in the lungs or post-lung site have been reported. In an attempt in solve the main points of dispute, with emphasis on the concept of stage specific immunity, the relevant problems will be examined by means of a quantitative autoradiographic method and also a quantitative autoradiographic-histologic method. The former method will be used as a scanning procedure to characterize the profile of the schistosomular migration in the skin and lungs and the latter method, to study the topographic relationship of the radiolabeling materials with the schistosomula, the percentage of the false negative and false positive foci of the schistosomula, and the number, relative locations, the vital conditions, and the cellular reactions of the schistosomula. The PR strain of S. manson and CF1 mice will be used in this study. Forty-eight kR or 12 kR will be used to expose the immunizing cercariae in order to attenuate them to die in skin or in lungs. The isotope to be used for labeling of cercariae is 75Se. The number of immunization is 1 or 5. The criteria for the attrition of the immunizing or the challenge cercariae in the skin of the experimental animals are: 1) the number of schistosomula in the relevant period of day 1 should be significantly less than that in the controls; 2) marked cellular reactions and deteriorated schistosomula should be found in the skin; and 3) the number of schistosomula in the peak day of the lungs should be significantly less than that of day 1. Naive mice infected with normal cercariae will serve as control. Four groups of mice will be used for the study of the attrition of 48 kR and 12 kR-irradiated cercariae. Another four groups of mice will be used for the study of the site of challenge cercariae in mice immunized with those two groups of irradiated cercariae. The percentage attrition of 48 kR and 12 kR-irradiated cercariae in the skin, lungs and liver during the first and fifth immunization will be studied, and a comparison of the percentage attrition of challenge cercariae in the skin, lung and liver of mice immunized once or five times with 48 kR or 12 kR-irradiated cercariae will be made. By comparing the results among 9 groups of mice, the problem concerning the site of attrition will be clarified.